In this section, JK Rowling told Harry Potter's past to make us know the cause of all these events.

Harry Potter and his friends began to search for Voldemort's remaining Horcruxes with the help of Goblin. Harry Potter finally returned to Hogwarts and gather friends to attack Voldemort.

When all has been successfully destroyed Horcruxes, Voldemort's impatience began to attack Hogwarts, and asked to submit to a duel with Harry Potter. Can Harry Potter defeated Voldemort? The story is that the past is behind Harry Potter? Everything will be explained in this movie ..
Just watch it! :)